An identifier (ID) locator separation technology has been available as a technology in which identifiers (addresses) on the Internet are managed separately in a core network and an access network to reduce the size of a routing table. A typical example of the ID-locator separation technology is Locator/ID Separation Protocol (LISP) and, in recent years, the standardization thereof is under consideration at the Internet Engineering Task Force (IETF).
FIG. 1 is a block diagram illustrating an overview of a network system employing the LISP in related art. This network system employing the LISP includes a core network 1 and one or more access networks 2 (three access networks 2a, 2b, and 2c are illustrated in FIG. 1 by way of example), which are coupled to the core network 1. The core network 1 in this case is, for example, a backbone network. The access networks 2 are, for example, user networks (networks at corporations or the like). As illustrated in FIG. 1, the core network 1 in the LISP includes, at least logically, a data plane 11 (a data-system communication network) and a control plane 12 (a control-system communications network).
Access lines of the access networks 2 are accommodated in corresponding transfer apparatuses (relay apparatuses), which are called “edge nodes 4”. The edge nodes 4 are, in turn, coupled to the data plane 11 in the core network 1. The access networks 2 are accommodated in the respective different edge nodes 4. FIG. 1 illustrates an example including an edge node 4a that accommodates an access line from a host 3a in the access network 2a, an edge node 4b that accommodates an access line from a host 3b in the access network 2b, and an edge node 4c that accommodates an access line from a host 3c in the access network 2c. 
Herein, identifiers (IDs) represent the identifiers of the hosts 3 that couple to the corresponding access networks 2. In FIG. 1, the ID of the host 3a, the ID of the host 3b, and the ID of the host 3c are assumed to be ID1, ID2, and ID3, respectively. Locators (LOCs) are identifiers indicating, in the core network 1, the locations of the hosts 3 that couple to the access networks 2 and correspond to, in the core network 1, the identifiers of the edge nodes 4. In FIG. 1, the LOC of the edge node 4a, the LOC of the edge node 4b, and the LOC of the edge node 4c are assumed to be LOC1, LOC2, and LOC3, respectively. The IDs may be called endpoint identifiers (EIDs) and the LOCs may be called routing locators (RLOCs). Since IP addresses are typically used as the IDs and LOCs, IP addresses are also used herein but other identifiers may also be used as the IDs and LOCs.
In the example in FIG. 1, the ID2 is assumed to be an IP address “A.B.10.11” (A and B are each an arbitrary integer from 0 to 255). The access network 2b is assumed to have a mask length of 16 bits. In this case, the address range of the access network 2b is “A.B.0.0/16”. In FIG. 1, the ID3 is assumed to be an IP address “C.D.128.22” (C and D are each an arbitrary integer from 0 to 255). The access network 2c is also assumed to have a mask length of 24 bits. In this case, the address range of the access network 2c is “C.D.128.0/24”.
In the LISP, in the core network 1 (that is, on the data plane 11), packets are relayed based on the LOCs and, in the access network 2, packets are relayed based on the IDs. In order to implement the packet relay in the LISP, one or more management servers 6 that manage relations between the IDs and the LOCs are provided at a gateway or gateways of the core network 1 (that is, the control plane 12). In FIG. 1, there are provided a management server 6a for the edge node 4a, a management server 6b for the edge node 4b, and a management server 6c for the edge node 4c. 
In this case, although the IP address, which is the ID of the individual host 3, may be registered with the corresponding management server 6, an address range of aggregated IDs of the hosts 3 below the management server 6 (that is, a group of IP addresses with masks) may also be registered. In the latter case, the number of entries registered can be reduced. For example, as illustrated in FIG. 1, “A.B.0.0/16”, which is the address range of the access network 2b below the edge node 4b, and the LOC (LOC2) of the edge node 4b are registered with the management server 6b in association with each other. Also, “C.D.128.0/24”, which is the address range of the access network 2c below the edge node 4c, and the LOC (LOC3) of the edge node 4c are registered with the management server 6c in association with each other.
A basic operation of the LISP will be described with reference to FIG. 1. An operation of a case in which the host 3a (ID1) transmits data to the host 3b (ID2) will be described by way of example. The host 3a transmits, to the access network 2a, a packet containing data to which a header designating the ID (ID2) of the host 3b as its destination address is added (this packet is referred to as a “user IP packet”). The user IP packet is transmitted from the host 3a to the edge node 4a directly or via a router (a relay apparatus, not illustrated) (S1). Upon receiving the user IP packet, the edge node 4a encapsulates the user IP packet and transmits, through the data plane 11, the encapsulated packet to the edge node 4b that accommodates the host 3b that is the destination (ID2) of the of the user IP packet. At this point, the edge node 4a queries about the edge node 4b, which accommodates the host 3b, via the management server 6a on the control plane 12, as appropriate.
Now, a detailed description will be given of processing from when the edge node 4a receives the user IP packet until it encapsulates the user IP packet and transmits the encapsulated packet. When the edge node 4a does not know the address (LOC) of the edge node 4b that accommodates the host 3b that is the destination of the user IP packet (this case will be specifically described below), the edge node 4a transmits a LOC request message for querying about the LOC corresponding to the host 3b (ID2) to the management server 6a (S2). Upon receiving the LOC request message, the management server 6a relays the LOC request message to the management server 6b that manages the LOC2 corresponding to the destination address (ID2) contained in the LOC request message. The LOC request message is relayed on the control plane 12 directly or via control-system relay nodes 7 (relay apparatuses) as illustrated in FIG message is subjected to route control (routing), which is not described in this case, based on a control-system route table illustrated in FIG. 1. Upon receiving the LOC request message, the management server 6b recognizes the LOC2 corresponding to the destination (ID2) requested by the LOC request message, on the basis of the above-described registered address relation. The management server 6b then transmits, to the edge node 4a that is the source of the LOC request message, a LOC response message that contains the ID2 and the LOC2 related with each other (S4).
Upon receiving the LOC response message, the edge node 4a regards the user IP packet, received from the host 3a, as payload to generate an encapsulated packet (a LISP packet) in which a new IP header is added to outside the payload and transmits the encapsulated packet to the core network 1 (the data plane 11). On the basis of the received LOC response message, the edge node 4a sets the LOC2 for the destination address in the newly added IP header. The LISP packet is transferred on the data plane 11 directly or via relay nodes 5 (relay apparatuses) as illustrated in FIG. 1 and arrives at the edge node 4b (LOC2) (S5). The relay of the LISP packet is subjected to route control (routing), which is not described in this case, based on a route table (not illustrated). The edge node 4b removes (decapsulates) the outer IP header from the LISP packet and transmits the resulting user IP packet to the access network 2b. The destination address in the user IP packet is the ID2 set by the host 3a. Thus, the user IP packet is relayed from the edge node 4b directly or via a router (a relay apparatus, not illustrated) and arrives at the host 3b (ID2) (S6). As a result of the above-described processing, the transmission of the data from the host 3a (ID1) to the host 3b (ID2) on the basis of the LISP is completed.
In the LISP, the addresses (IDs) in the access networks 2 and the addresses (LOCs) in the core network 1 are separately managed as described above. That is, when viewed from the core network 1, the addresses (IDs) of all hosts 3 in one access network 2 are aggregated into the core network 1 address (LOC) of the edge node 4 to which the access network 2 couples. In other words, the relay nodes 5 in the core network 1 do not have to be conscious about the address system in each access network 2. Thus, the use of the LISP makes it possible to reduce the size of routing tables in the core network 1.
In general, when a LOC request message is transmitted each time a packet (a user IP packet) is received from the host 3a, the amounts of load on both of the edge node 4a and the entire network system are large. Thus, the edge node 4a holds a cache table (referred to as a “LOC cache table”) in which the IDs and LOCs of the destination hosts 3 are related with each other. For example, in FIG. 1, “A.B.128.22/24, LOC3”, which is an entry corresponding to the host 3c (ID3), is registered in the LOC cache table in the edge node 4a. Upon receiving a user IP packet from the host 3a, the edge node 4a searches the LOC cache table by using the destination address of the user IP packet as a search key. When the destination address does not match any of entries in the LOC cache table (this situation refers to a case in which the edge node 4a does not know the LOC), the edge node 4a transmits a LOC request message in order to query about the LOC, as described above. On the other hand, when the destination address matches any of the entries in the LOC cache table (this situation refers to a case in which the edge node 4a knows the LOC), the edge node 4a transfers the received packet to the edge node 4 by using the LOC in the matching entry without transmitting a LOC request message. With this arrangement, it is possible to reduce various types of load involved in query of the LOCs.
Examples of the related art include Japanese National Publication of International Patent Application No. 2002-506302 and Japanese Laid-open Patent Publication No. 2004-23450.
Other examples of the related include D. Farinacci, V. Fuller, D. Meyer, D. Lewis, cisco Systems, “Locator/ID Separation Protocol (LISP) draft-ietf-lisp-22”, Feb. 12, 2012 and V. Fuller, D. Farinacci, D. Meyer, D. Lewis, Cisco “LISP Alternative Topology (LISP+ALT), draft-ietf-lisp-alt-10”, Dec. 6, 2011.